The present invention relates to electroabsorption (EA) modulators and, more particularly, to a tandem EA modulator structure where one modulator is used as a phase modulator and the other is used as an amplitude/intensity modulator to provide for tunable chirp at the tandem modulator output.
For short distance or low bit rate optical communication systems, a directly modulated laser can be used as a transmitter. However, this technique has many limitations that preclude its use for longer distances or higher bit rate applications. In particular, a directly modulated laser is limited in terms of its bandwidth, large frequency chirp and low extinction ratio (difference between a logic xe2x80x9c1xe2x80x9d and logic xe2x80x9c0xe2x80x9d). These fundamental limitations have been the driving force behind developing xe2x80x9cexternalxe2x80x9d optical intensity modulators for the transmitter laser sources. Optical intensity modulators can be categorized into two main types based on the physical phenomenon that they use to modulate the light. One category contains modulators that rely on the electro-optic effect to change the effective index of a waveguide and modulate the phase of an optical signal. This type of modulator typically employs a Mach-Zehnder interferometer geometry to a convert the phase change into an intensity modulation. The other category of modulators is based on the electroabsorption (EA) effect, which changes the absorption in an optical waveguide to modulate the intensity of a lightwave passing through the modulator. There are a number of advantages and distinctions associated with each of these categories of modulators, but in general they both offer substantial benefits over direct modulation. Among the benefits are low or negative chirp, high extinction ratio and bandwidths that are wide enough to support ever-increasing high data rates.
In particular, EA modulators employ the quantum confined Stark effect (QCSE) to change the absorption in a semiconductor optical waveguide, where this change then modulates the intensity of the light transmitted through the device. This change in the absorption properties also produces, however, an undesirable change in the index of refraction along the waveguide, leading to a shift in the frequency of the optical wave propagating through the device (referred to as xe2x80x9cfrequency chirpxe2x80x9d). As is well known in the art, uncontrolled frequency chirping can result in degradation of the transmission performance for fiber optic-based communication systems. That is, for a device exhibiting negative chirp (a downward shift in frequency), the maximum transmission distance in single mode fiber will be substantially reduced. However, for a device exhibiting optimized xe2x80x9cpositivexe2x80x9d chirp, the transmission distance can actually be increased. In conventional EA modulators, a positive chirp can only be realized over a narrow wavelength range. Even in this mode, the device must be biased to the point where it exhibits a large insertion loss.
Thus, a need remains in the art for an EA modulator that exhibits an improved chirp characteristic over the prior art and, preferably, exhibits a tunable (both positive and negative) frequency chirp over a relatively wide wavelength range.
The need remaining in the prior art is addressed by the present invention, which relates to electroabsorption (EA) modulators and, more particularly, to a tandem EA modulator structure.
In accordance with the present invention, a tandem EA modulator comprises a first modulator element that functions as a phase modulator, and a second modulator element that functions as a conventional (amplitude/intensity) EA modulator. The conventional EA modulator element is driven by the usual xe2x80x9cdataxe2x80x9d signal and a xe2x80x9cchirp tuning signalxe2x80x9d is used as the drive signal input to the phase modulator element of the tandem arrangement. In its simplest form, the complement of the data signal can be used as the chirp tuning signal. Alternatively, a sinusoidal signal that is synchronous with the data signal can be used as the input for the phase modulator element, or a sinusoidal, synchronous complement of the data signal may be used. In any version, the phase
changing output of the phase modulator component is used to compensate for the inherent chirp of the conventional EA modulator by modifyng either the amplitude or the bias point for the phase modulator drive signal thus enabling the chirp of the signal exiting the EA modulator to be tuned from negative to positive across a relatively wide wavelength range.
In one embodiment, the phase modulator and intensity modulator are formed along the same optical waveguide as a monolithic device. In an alternative arrangement, a different quantum well structure, with a higher band gap energy, is used for the phase modulator element of the tandem structure.
The tunable chirp tandem EA modulator of the present invention can be used as an external modulator with a laser gain section, or with a semiconductor amplifier as part of a larger integrated optical arrangement. Indeed, various other optical components can be used in association with the tunable chirp tandem EA modulator and, in fact, the modulator sections may be separately disposed as input and output devices in association with various optical devices (e.g., the phase modulator element disposed at the input of a semiconductor optical amplifier and the intensity modulator element disposed at the output of the same semiconductor optical amplifier).
Other and further embodiments and aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.